Silica-based glass, such as soda-lime glass, is prevalent in the manufacture of glass containers and other products. Formation of the glass melt typically involves mixing the various glass-forming components at elevated temperature. The glass typically has a residence time in a furnace on the order of twenty-four hours to fully dissolve the raw materials of the batch composition and to refine the glass by driving off gases. The gases must be driven off ultimately to produce a solidified glass product without entrained gas bubbles. (The process of removing bubbles in molten glass is called “refining.”) In addition to being undesirably slow, this in-furnace process involves a large amount of space and high energy input.
A general object of the present disclosure is to provide a process for making silica-based glass, which is more rapid and requires less energy input than conventional processes.
The present disclosure embodies a number of aspects that can be implemented separately from or in combination with each other.
A process for making a silica-based glass, in accordance with one aspect of the present disclosure, includes: (a) reacting at least one glass network former and at least one glass network modifier to form a glass precursor melt, the glass precursor melt being at a temperature in the range of 900 C to 1700 C and having a viscosity of not more than 3 Pa·s, and (b) refining the glass precursor melt. Target viscosity typically would be in the range of 1 to 3 Pa·s, with a value of 2 Pa·s being an optimum target. The term “glass network former” is employed in its usual sense in the art as referring to oxides and non-oxides capable of forming and being incorporated into a random glass network or lattice. Silica (SiO2), alumina (Al2O3) and boric oxide (B203) are three examples of glass network formers. The term “glass network modifier” likewise is employed in its usual sense to refer to oxides within the glass that do not participate in forming the glass network structure and force the glass network to form around the modifier. Examples include the oxides of the alkali metals (sodium, lithium and potassium) and oxides of the alkaline earth metals (calcium, magnesium and barium).
Either or both steps (a) and (b) can be carried out under reduced pressure to enhance refining. The refined glass precursor melt preferably is mixed with additional glass network former materials including silica (SiO2) and/or non-gas releasing network modifiers to form the desired glass melt composition. Cullet and/or minor ingredients such as colorants can be added at this stage. Step (b) can include stirring.
The term “gas releasing” is used to describe chemical forms of an element that evolve gases upon decomposition, including the carbonate, bicarbonate, sulfate, hydrate, hydroxide, nitrate, chloride or acid form(s) of a given element. The term “non-gas releasing” is used to describe chemical forms of an element that do not evolve gases upon decomposition, including the oxide, sulfide and elemental forms of a given element.
A process for making a silica-based glass in accordance with another aspect of the present disclosure includes: (a) forming a low-viscosity sodium-calcium-silicate solution in liquid phase while releasing gaseous reaction products, and (b) mixing the sodium-calcium-silicate solution with additional material including silica to form a soda-lime glass melt. Step (a) is carried out by (a1) melting, reacting and refining calcium carbonate, soda ash and silica, or (a2) by melting, reacting and refining salt and silica in the presence of water followed by addition of calcium carbonate. The sodium-calcium-silicate liquid solution intermediate phase product of step (a) preferably has a viscosity of not more than 3 Pa·s to promote release of gaseous reaction products such as carbon dioxide in step (a1) or hydrogen chloride in step (a2). Step (a) and/or step (b) can be carried out under reduced pressure further to promote release of gaseous reaction products. Step (b) can include stirring.
A process for making a glass precursor melt in accordance with a third aspect of the disclosure includes mixing at least one glass network former with at least one glass network modifier, and refining the glass precursor melt by performing at least part of the mixing step at elevated temperature and under reduced pressure to promote release of gases. Preferably, all of the gas releasing glass network modifiers and gas releasing glass network formers that are required to achieve the desired glass product melt composition are added to the precursor melt for refining in step (a). Additional non-gas releasing glass network formers and non-gas releasing glass network modifiers are mixed with the glass precursor melt to form the desired glass product melt composition. Cullet and/or minor ingredients such as colorants can be added.